Yevgeny Urlashov stands in front of his mayoral campaign poster in downtown Yaroslavl, Jan. 24, 2012. Urlashov, has been detained on suspicion of corruption and extortion, just a few months before he was to head an opposition ticket in upcoming regional elections.

The Kremlin's Investigative Committee said he and two aides are under suspicion of soliciting a $425,000 bribe from a private company in exchange for lucrative contracts to perform municipal services. Urlashov says his accuser is a prominent member of the pro-Kremlin United Russia party.

That electoral upset triggered alarm bells in United Russia headquarters, and put authorities on notice that pro-Kremlin candidates are vulnerable to defeat – despite the concentration of official resources deployed on their behalf in Russia's system of "managed democracy" – especially when the opposition is well organized and fields an experienced and capable candidate.

Russian news reports say Urlashov was detained at a police checkpoint on Tuesday night. Plainclothes officers dragged him from his car and placed him in detention before proceeding to search his home and offices. He was expected to head Civic Platform's candidate list for the regional legislature in September.

"Putin's regime has consistently tried to avoid overtly pressuring and prosecuting people for being political adversaries," says Masha Lipman, editor of the Moscow Carnegie Center's Pro et Contra journal.

Instead, she says, the government has become a "mastermind of selective law enforcement," using anti-corruption charges to hobble opposition figures, creating and changing the law at will to service its attacks.

"It's also important to remember that in Russia's shadow economy, informal practices are ubiquitous. It's almost impossible to do any kind of business without engaging in something that's a legal gray area," she says. "That makes virtually everyone vulnerable.... Hence the authorities are able to decide who to prosecute, and how hard to go after them. It's a useful way to intimidate everyone and remove unwanted political players, while at the same time claiming that it's really all about fighting crime and corruption."

But the arrest of Urlashov can be read as a clear challenge to Prokhorov to tone down Civic Platform's ambitions in advance of the September elections.

It's not clear how Prokhorov will react. But on Wednesday he insisted in his LiveJournal blog that he intends to go ahead and attend a Civic Platform conference in Yaroslavl this weekend and lend all possible support to Urlashov.

"The demonstration seizure of the elected leader of the city [Yaroslavl] is a blow to the civil liberties of every Russian citizen," Prokhorov wrote.

"It's obvious that this night time "mask show" [by balaclava-wearing cops] was arranged with the sole purpose of intimidating Yevgeny [Urlashov] personally, as well as all independent politicians and simply active citizens of Russia," he wrote.